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Why is pressure so important? In tunnel kilns, changing the kiln pressure is probably the fastest way to modify the firing and cooling characteristics of the entire kiln. Clients frequently ask me about the "correct" pressure for their tunnel kiln-without fully understanding the importance of kiln design, sensor location and sensor elevation.
Pressure controls the primary and fundamental airflow within the tunnel kiln. It has a dramatic influence on temperature uniformity, soaking time and energy consumption. Measurement and control of pressure is imperative to maintaining consistently good firing characteristics.
Basic Pressure CharacteristicsPressure within a tunnel kiln varies along its length, and is elevation- and temperature-dependent. Pressure is important because it influences airflow direction within the kiln and affects air infiltration and outflow. Airflow direction is always from positive to less positive, and it is important to remember that kiln pressure is measured relative to the air pressure outside of the kiln.
In Figure 1, it is evident that the kiln pressure is relatively negative within a few feet of the entrance of the kiln and then gradually rises; the pressure becomes positive through the firing and cooling zones, and is "neutral" at the kiln exit. As previously stated, this pressure curve is only relative to the pressure in the room where the kiln is located. In fact, there is no negative pressure-only more positive or less positive areas. The absolute pressure within the kiln actually varies hour by hour with normal barometric changes in the atmosphere. Under proper control conditions, however, the relative pressure within the kiln stays the same.
Finding a BalanceOnce the basic concept is understood, how do you adjust the kiln pressure to provide the proper balance between temperature uniformity, temperature control, kiln car overheating and energy consumption? The first step is to develop a set of readings on your tunnel kiln. Select a manometer with sufficient sensitivity-0.001 in. resolution-so you can develop an internal pressure curve of the kiln. Try to measure the pressure in the kiln every 20 ft, and measure the pressure at the same elevation (preferably close to the kiln car base).
At the same time, measure the oxygen level at each of these locations, as this supplemental data will be quite useful as we begin the analysis of changes that must be made. This data can be used to help improve the operational characteristics of the kiln.
Because pressure controls the primary and fundamental airflow within the tunnel kiln, it has a dramatic influence on temperature uniformity, soaking time, and energy consumption. Measurement and control of pressure is imperative to maintain consistently good firing characteristics. Adjustment of the kiln pressure curve must take into account the whole kiln-not just one position. When changing any input or exhaust, whether it is cooling or heating, it is necessary to consider the impact and make the appropriate balance adjustments.
Editor's note: Future installments of this column will discuss additional factors related to kiln pressure.